A binaural reproducing method has heretofore been known as an approach for providing better direction sensation of sound image or outside head localization sensation when audio signals are reproduced by headphones fitted to the head of a listener so that a pair of headphones are located in the vicinity of both ears.
An audio reproducing system adopting this binaural system preliminarily applies a given signal processing to the audio signals reproduced by headphones as is described in, for example, specification of Japanese Patent Publication Sho 53-283.
The direction sensation of sound image and outside head localization sensation and the like depend upon tile differences in volumes, times and phases of sounds listened by left and right ears.
The signal processing aims at causing in an audio output reproduced by the headphones, audio effects equivalent to those caused by the difference in distances between sound sources, that is, speaker systems and right and left ears of a listener and reflections and diffractions in the vicinity of the head of the listener when audio reproducing is performed, for example, by speaker systems remote for the listener. Such a signal processing is performed by convolution-integrating left and right ear audio signals with impulse responses corresponding to the above-mentioned audio effects.
Since the absolute position of the sound image is not changed even if the listener moves or turns his or her head when audio reproducing is performed by speaker systems remote from the listener, the relative direction and position of the sound image that the listener senses are changed. In contrast to this, since the headphones is turned together with the listeners head if the listener turns his or her head when audio reproducing is performed by a binaural method using headphones, the relative direction and position of the sound image which the listener senses are not changed.
If binaural reproducing is performed by using headphones in such a manner, a sound image is created in the head of a listener due to differences in displacement of the sound image relative to a change in direction of the listener's head. Therefore, it is difficult to locate the sound image in front of the listener. Furthermore, the front sound image has a tendency to lift up.
Accordingly, an audio signal reproducing system which detects a change in the direction of the listener's head and changes the modes of the signal processing based upon a result of the detection for providing a good front localization sensation in headphones has heretofore been proposed as is disclosed in Japanese Unexamined Patent Publication No. Sho 42-227 and Japanese Examined Patent Publication No. 54-19242. In such an audio signal reproducing system, a direction detecting device such as gyro compass and magnetic needle is provided on the head of the listener. A level adjusting circuit and a delay circuit and the like for processing the audio signals are controlled based upon a result of detection from the direction detecting device so that a sound image sensation which is similar to that of the audio reproducing using speaker systems remote from the listener is obtained.
In the prior art binaural reproducing system in which headphones are provided with a direction detecting device comprising a gyrocompass, an excellent sound image can be obtained by controlling the content of the signal processing which is applied to the audio signals depending upon changes in direction of the listener's head.
In order to control the content of the signal processing applied to the audio signals depending upon a change in direction of the listener's head, it is necessary to preliminarily measure the impulse responses, that is, transfer characteristics corresponding to audio effects given to audio signals of left and right ears for each predetermined rotational angle and to store a great amount of information on the transfer characteristics. The information is read from the storing means depending upon the change in direction of the head. The audio signal will be subjected to a necessary convolution-integration processing in real-time.
The present invention was made under such circumstances.
It is an object of the present invention to provide an audio signal reproducing apparatus having a simple structure using storing means having a low storing capacity which is capable of performing a binaural reproduction for providing a very natural localization of a sound image in which the positions of virtual sound sources are not changed by headphones even if a listener moves by reducing the amount of information on transfer characteristics from virtual sound sources necessary for binaural reproduction of audio signals with the headphones to both ears of the listener.